Differential current-mode interfaces are currently used in computer systems to connect a plurality of storage devices, such as disks and tapes, to an intelligent host interface, such as a storage controller. Such differential current mode interfaces, also known as the Director to Device Connection (DDC) generally include a cable formed of one or more wire pairs each of which runs from the storage controller to a respective storage device, in a "daisy chain" configuration. Each pair of wires, forming the cable, is terminated in its differential, characteristic impedance at both ends. Drivers and receivers used with these interfaces are designed such that the impedance they present to the interface is much higher than that of the termination so that they do not substantially influence the overall interface impedance as seen from any point on the interface.
These interfaces may, in addition to the cable, include card wiring and wiring internal to the machine leading to external connectors and terminator functions within the entire system as well as any wiring interconnecting multiple machines. Any of these wires or components can suffer faults, such as opens or shorts to ground, and those potions outside the machine are particularly subject to such faults due to damage during installation and use. Since these interfaces are differential, they can function nominally even with one of the two wires faulted. However, it is common, under these conditions, for the interface to perform intermittently or at degraded transmission rates due to multiple retries. This impacts the accessibility of the data to the user and can be difficult to diagnose.
Prior art fault detectors, of which U.S. Pat. No. 4,782,300 assigned to the same assignee of the present invention is typical, have two known weaknesses. The first is that they ignore characteristics of the interface such as common-mode voltage shifts and cable resistance and depend to some degree on the processing parameters used to fabricate the transceiver.
More specifically the prior art fault detector, taught in the referenced patent, operates on the principle that the voltage at the biased end of the cable depends strongly on the status of the cable and the terminating resistor at the far end. Unfortunately, for certain values, the voltage at the biased end of any wire in the cable will approximately double if the wire is open regardless of the common-mode voltage. At the same time the absolute voltages for the normal, shorted and open cases are a strong function of the common-mode voltage. The prior art ignored these conditions and thus will, at times, diagnose good lines as defective and defective lines as good.
Other more cumbersome methods to isolate such fault have also been used. One such method requires a repairman manually installing a special DDC wrap plug at various point along the DDC and repeated interaction on the part of the diagnostician with the service panel. This procedure leads to long down times and considerable effort on the part of the diagnostician and thus is not desirable.
Accordingly, there now exists a need for a cable detection system that avoids all the above described problems associated with the detection of opens and shorts in cables used with such differential common-mode interfaces.